Possible bug in reported force constants after vibrational analysis
Matthias Krack
matthia... at psi.ch
Sun May 25 10:38:17 UTC 2014
Hi Leopold,
I think the code is correct, since the eigenvalues epsilon of the Hessian
returned in H_eigval2 after its diagonalisation are equivalent to
SQRT(konst/rmass). The frequencies, however, are proportional to
SQRT(epsilon), i.e. SQRT(H_eigval2).
Matthias
Am Freitag, 23. Mai 2014 16:11:57 UTC+2 schrieb Leopold Talirz:
>
> Dear developers,
>
> during an MD tutorial using CP2K I was looking at the force constants k
> reported after vibrational analysis. Below you find an example for the
> water molecule
>
> VIB| 1 2 3
> VIB|Frequency (cm^-1) 1589.789609 3637.026185
> 3741.860069
> VIB|Intensities 0.000000 0.000000
> 0.000000
> VIB|Red.Masses (a.u.) 1.083688 1.044444
> 1.081600
> VIB|Frc consts (a.u.) 0.017723 0.467889
> 0.542862
>
> I thought that the force constants are defined such that k = mu * omega^2,
> where mu is the reduced mass and omega the vibrational frequency (?). But
> this is obviously not the case.
>
> Looking into vibrational_analysis.F , the formula
>
> konst(i) =
> SIGN(1.0_dp,H_eigval2(i))*2.0_dp*pi**2*(ABS(H_eigval2(i))/massunit)**2*rmass(i)
>
> for the force constant on line 406 (r14024) involves H_eigval2^2, which is
> proportional to omega^4 instead of omega^2.
> Is this a bug?
>
> Best,
> Leopold
>
>
>
>
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